The experiments proposed in this research proposal are aimed at understanding the development and circuitry of one of the major descending pathways involved in the regulation of motor function, the rubrospinal tract. Much remains to be learnt about the functional anatomy of these projections, and the mechanisms by which growing axons project specifically to their spinal targets during development. Although lesion studies have implicated the rubrospinal tract in the control of skilled forelimb movements, the specific circuitry which underlies the rubrospinal contribution to motor output has yet to be determined. Intriguingly, anterograde tracer injections in rodents suggest that there are direct rubrospinal terminations on restricted subpopulations of motor neurons within the spinal cord. In this project, I will use genetic techniques to specifically and completely label the rubrospinal population, enabling the fine mapping of rubrospinal terminations onto spinal inteneurons and motor neurons in the mouse. Specific Aim 1 will characterize the molecular identity and anatomical organization of functionally distinct populations of neurons within the red nucleus. Molecular markers of the rubrospinal tract will be used to create genetic tools with which to visualize rubrospinal projections and synaptic terminations. Specific Aim 2 will identify rubrospinal targets within the spinal cord, focusing on projections onto specific subpopulations of motor neurons, and identifying distinctions in circuitry between the ipsilateral and contralateral rubrospinal projections. It will also provide an accurate timecourse of the development of the rubrospinal circuit. The results of this project will provide a defining analysis of the development and patterning of the circuitry which ultimately determines the ability of the rubrospinal tract to regulate motor output. PUBLIC HEALTH RELEVANCE: Elucidation of the molecular mechanisms involved in the patterning of rubrospinal development and circuitry during development will provide insight into potential mechanisms of regeneration and repair following injury. Furthermore, the genetic tools generated from this project will enable the study of rubrospinal dysfunction in rodent models of neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS).